1. Field of the Invention
This invention relates in general to an apparatus for reducing acoustic noise in disk drives, and more particularly, to a spindle motor shaft having predetermined geometrical features that may be adjusted to tune the torsion mode frequency of the spindle motor shaft away from the driving frequency of the motor.
2. Description of Related Art
Disk drives can produce a relatively large amount of acoustic noise. For example, causing a storage disk in a disk drive to spin at high speeds requires exciting the motor at high frequencies. These high frequencies in turn excite the stator windings and laminations. This energy is then transmitted to the bearings, disks, casting, and the actuator assembly.
In many disk drive designs, a disk drive spindle motor has a stator that is attached to the spindle motor shaft. The stator supports windings through which current is driven to generate a magnetic field. This magnetic field is used to exert a force on magnets attached to the rotor. The rotor also has a hub attached to it whereon at least one disk is attached. The spindle motor shaft is attached to the disk drive device enclosure. This stator/shaft structure has natural modes of vibration. One of the fundamental modes of vibration for this structure is torsion. When motor drive forces are generated by current flowing through the stator windings, a force is exerted on the magnets of the motor and an opposite force is exerted on the stator. This causes the stator and shaft to oscillate about the longitudinal axis of the shaft in its torsional mode of vibration. Since one or both ends of the shaft are fixed to the device enclosure, the shaft acts like a torsional spring while the stator acts as a flywheel inertia.
When the magnetic forces in an operating motor act to excite the torsion mode of the shaft/stator assembly, reactively large vibrations in the shaft/stator assembly can result if a component frequency of the magnetic force is coincident or near the torsion mode frequency. The energy from the resulting vibration flows from the shaft/stator assembly into the device enclosure to which it is attached. The device enclosure then acts like a speaker with significant surface area to produce acoustic emissions from the drive.
Many approaches have been used to reduce the source of acoustic noise in disk drives. Constrained-layer damping has been used on device enclosures to reduce acoustic noise. For example, constrained-layer damping material has been applied to the end of a spindle motor shaft. Further, it is known that applying a damping material between the inner diameter of the stator and outer diameter of the shaft decouples the stator vibrations from the shaft.
One approach for reducing acoustic noise is disclosed in U.S. Pat. No. 5,282,100, issued Jan. 25, 1994, to Boyle et al., entitled "DISK DRIVE WITH REDUCED ACOUSTIC NOISE", and incorporated herein by reference. Boyle et al. disclose providing mechanical isolation and sound dampening between the cover and interior chamber of a disk drive enclosure. The acoustic noise otherwise emanating away from the inner cover is attenuated. However, the amplitude of the acoustic noise at the source is not reduced.
Another approach for reducing acoustic noise in disk drives is disclosed in Japanese Patent 06-52626, invented by Yotaro Sanada, entitled "SPINDLE MOTOR FOR MAGNETIC DISK DEVICE", and incorporated herein by reference. Sanada discloses interposing O-rings between the armature and the shaft to suppress the transmission of vibration from the shaft to the housing and cover.
Yet another approach for reducing vibration of a spindle shaft is disclosed in Japanese Patent 01-8574, invented by Iizuka et al, entitled "DISK STORAGE DEVICE", and incorporated herein by reference. Iizuka et al. disclose using a support arm to suppress the lateral vibration of the spindle shaft.
IBM Technical Disclosure Bulletin, Vol. 37, No. 10, October 1984, pp. 205-208, entitled "REDUCING MOTOR EXCITATION TO CASTING/BEARINGS", incorporated herein by reference, discloses reducing motor vibrations from exciting structural modes by interposing a dampening material between the spindle shaft and the stator. The interposition of the dampening material between the stator and the shaft reduces the amplitude of the excitations transmitted to the bearings and to the structure. The vibration isolation reduces the spindle NRRO and file acoustics. Anisotropic properties are desirable for the damping material because the radial components of the excitation forces are much greater than the axial components. Thus, the dampening material provides high damping characteristics in the radial direction.
IBM Technical Disclosure Bulletin, Vol. 36, No. 9B, September 1993, pp. 41-42, by Ando et al., entitled "LOW-NOISE SPINDLE MOTOR FOR SMALL HARD DISK DRIVE", incorporated herein by reference, discloses adjusting the ball pitch circle diameter of bearings in the spindle motor to prevent beating tones in disk drives.
Nevertheless, none of these prior approaches have focused on adjusting the torsion mode frequency to eliminate acoustic noise in disk drives. At the root of the acoustic mechanism is the forcing function. The magnetic forces in the motor are very complicated and have a rich frequency content. One of the largest spectral components of the magnetic force is the commutation or "switching" frequency of the motor driver. When the frequency of the shaft/stator torsion mode is near or coincident with particular magnetic motor force components, the result can be a relatively loud pure-tone acoustics problem for the file (i.e. a continuous "screaming" type sound).
For example, for a 9-slot 8-pole (9.times.8) motor, there are 24 "switches" per mechanical revolution. A 9.times.8 motor spinning at 7200 RPM (120 Hz) has a commutation frequency of 2880 Hz (f=24.times.120=2880 Hz). Furthermore, the commutation frequency generally has harmonics above the fundamental (i.e., there will typically be energy at 2f, 3f, 4f, etc.). When the torsion frequency of the shaft/stator assembly is near the driving force frequency or one of its harmonic components, the sound pressure of the harmonic component becomes high and produces a characteristic "scream" or "whine."
The amplitude of the "whine" can be reduced by loosening or tightening the screws that attach the spindle to the device enclosure, and/or by damping the device enclosure with a constrained-layer damper (CLD). Loosening the screws has a positive effect because the torsion mode frequency is reduced in the process because the end conditions on the shaft become less "fixed". This moves the torsion mode frequency of the stator/shaft assembly away from the fixed excitation frequency. The constrained-layer damper has a positive effect by damping the "speaker" mechanism. However, these measures only work as an interim fix, since it is desirable to eliminate the constrained-layer damper and to reinstate the plan-of-record (POR) screw torque during manufacturing.
None of the prior attempts at damping or attenuating acoustic noise in disk drives discussed have focused on shaft designs which allow the torsion mode frequency of the shaft/stator assembly to be tuned and thus moved away from the driving force.
It can be seen then that there is a need for spindle motor shaft designs having tunable torsion mode frequencies for reducing acoustic noise in a disk drive.
It can also be seen that there is a need for a spindle shaft design having reduced torsional stiffness due to selected geometrical features.